LanguageInterface should not support setName

@Gabor, i was trying to solve this issue,but i have some doubts regarding implementation of filter languages().Since we have to remove setName() so do we need to override default language name as it was done by setName() ? I guess removing "$default->setName($this->t("Site's default language (@lang_name)", array('@lang_name' => $default->getName())));" this line would be fine.
Please tell me.Thanks.

I am working on it.

Removed SprintWeekend2015Queue by mistake.

I've made something, but i don't now if it's the solution. If not, please help me to get on the right way. Tanks!

I updated the Testcases, so there was a test case for the setName method.

Applies, tests pass. Looks good.

+++ b/core/lib/Drupal/Core/Language/LanguageManager.php
@@ -397,9 +397,9 @@ protected function filterLanguages(array $languages, $flags = LanguageInterface:
-      // Setup a language to have the defaults, but with overridden name.
...
+      // Setup a language to have the defaults
       $default = $this->getDefaultLanguage();
-      $default->setName($this->t("Site's default language (@lang_name)", array('@lang_name' => $default->getName())));

This is important - this allows sites to translate Site's default language. We shouldn't be removing this. I think Gabor was thinking we should be creating new language object here.

Thanks a lot, i hope i got it now. If not, please help me, i'ts all new to me :)

This clarifies the difference between Language and ConfigurableLanguage and it also makes Language a true value object because it is immutable. Nice.

This issue is a normal bug fix, and doesn't include any disruptive changes, so it is allowed per https://www.drupal.org/core/beta-changes. Committed 574b0eb and pushed to 8.0.x. Thanks!

Oops missed something - we should be moving LanguageInterface::setName to ConfigurableLanguageInterface since we are not removing ConfigLanguage::setName() - and we shouldn't be removing that.

Sure, I'll try..

I could need some help. I searched a lot and dived deeper and deeper into Drupal (that's the only good thing on that) but with no result.
The only thing that i think we should made is that we should implement a setUp() Method for the ConfigurableLanguageUnitTest. I've tried to setup there a Language Entity Manager (wich is needed in the label() Method of Entity.php) like it's been done in some other tests. But the Error still exists.

/**
 * {@inheritdoc}
 */
public function setUp() {
  $container = new ContainerBuilder();
  $container->set('entity.manager', $this->getMockBuilder('Drupal\language\ConfigurableLanguageManagerInterface')
    ->getMock());
  \Drupal::setContainer($container);
}

I had also problems to debug. I run the test alwas in my console with

php core/scripts/run-tests.sh --color --verbose --file core/modules/language/tests/src/Unit/ConfigurableLanguageUnitTest.php

but i don't know how to make test outputs like print_r / print / echo / var_dump etc. I've written this outputs always in a textfile wich isn't a good soloution..

As mentioned by @Cogax, we need the container to test setName/getName methods. Added new kernel test case which covers this.

Thanks for the review @Gábor Hojtsy. Here is an update...

This issue is a normal bug fix, and includes only a very small disruptive change that contrib should not have been relying upon, so it is allowed per https://www.drupal.org/core/beta-changes. Committed 4f776f6 and pushed to 8.0.x. Thanks!